can multiple users log into the Wii U at the same time?
It is not possible. I've wondered this for myself for Injustice: Gods Among Us and Skylanders Giants, but it would appear only one user can be logged in at a time, which is a really weird option in my opinion. It's probably because save files are tied to the user account, and the way they have it set up, it might do some funky stuff if both users are trying to access their save files at the same time.